Conditioning of sheeted cellulose pulp for acetylation and the like



Patented Mar. 22, 1938 CONDITIONING OF SHEETED CELLULOSE PULP FOR ACETYLATION AND THE LIKE George A. Richter, Berlin, N. 11., assignor to Brown Company, Berlin, N. IL, a. corporation of Maine No Drawing. Application April 4, 1936,

Serial N0. 72,837

17 Claims. (01. 2so-1o1 This invention relates to the conditioning of sheeted cellulose pulp for acetylatlon and deals more especially with sheeted wood pulp which can be acetylated with suflicient uniformity so that 5 the cellulose acetate prepared therefrom is soluble in the usual solvents toform solutions of substantial clarity and freedom from undissolved cellulosic residues. The present invention also improves the condition of sheeted pulp for the preparation of various cellulose derivatives or compounds, including cellulose xanthate, cellulose nitrate, etc., even though it is of especial utility in-the field of cellulose acetate preparation and will be stressed more particularly as applied in this field.

In preparing wood pulp in a wood pulp mill for sale to paper makers and chemical converters, it is the practice to sheet the pulp on pulp driers as a comparatively thick sheet known as pulpboard or drier sheet. The sheet, which may be of a thickness of about 0.03 to 0.10 inch, is either accumulated as large rolls or cut into large square sheets as it comes off in substantially dry condition from the pulp drier and these sheets are 5 stacked and baled tightly within a suitable wrapper immediately after cutting so that they may be conveniently handled, stored, and shipped. The rolls or baled sheets constitute an advantageous form of cellulose for such industries as viscose and nitrocellulose manufacture; and, by the practice of the present invention, they are rendered quite satisfactory for conversion into cellulose acetate.

- I have observed that sheeted wood pulp of the character described is apt to give trouble in the cellulose acetate industry in that cellulose acetate prepared therefrom tends to yield cloudy or turbid solutions even when the wood pulp is of.

high whiteness and purity, that is, consists practically entirely of alpha cellulose. I have determined that this trouble often arises from the manner'in which the pulp is ordinarily dried and shall indicate how the dried pulp may in ac cordance with the present invention be condi- 5 tioned for satisfactory acetylation.

substantially dry condition. In order to get as high production as possible from a pulp drier,

The sheet' coming from the drier drums is in the desired it isdesirable to circulate high temperature steam through the drier drums, that is, steam at a temperature upwards of 212 F. The water in the interior of the sheet is thus expelled or volatilized quickly through its surfaces. However, it is quite diflicult to drive the last percentage of water more especially from the interior of a thick sheet of the character described so that it is usually the case that the sheet as a whole containsabout 2% to 5% of water as delivered by the drier. 10 This residual water is not, however, uniformly distributed throughout the sheet. On the contrary, its exterior, which has been in contact with the hot drum surfaces is practically bone-dry, whereas its interior contains practically all the 15 residual .water. To the practically bone-dry condition of the exterior of the sheet is traceable the unsatisfactory results secured upon its acetylation, since the exterior fibers apparently-shrink unduly andacquire a dense or case hardened" o condition bespeaking passivity toward acetylating reagents. Moreover, the small percentage of water present in the interior of the sheet evidently tends to remain there by reason of the reluctance of the interior fibers to give up their 25 very small moisture content by merecapillarity. In any event, the storage of the sheets over a long period does. not restore their surface portions to the desired acetylating activity, Particularly when, as is usual commercial practice, they 30v are stored as bales or large rolls so that atmospheric moisture does not have access to the sheet surfaces. The exposure to the atmosphere for a substantial period of time of the surfaces of all the sheet'material consumed in commercial 35 scale operation presents too much difficulty to be given serious consideration.

, characterized by their substantial perfect clarity or freedom from haze. .The activation of the surface portions of the sheeted pulp is preferably eiiected continuously, that is, as the sheet is being delivered continuously from the. my end of the pulp drier, although it is possible to accumulate the sheet as rolls and to perform the activating treatment on the surfaces of a sheet 55 2 v s,11a,11s

as is progressively unwound from a roll. There are various ways of activating the surface portions of a sheet pursuant to the present inthat the 'surface portions -oi' the redried sheet as well as its interior contain residual moisture 15 in the amount oLsay, about 2% to 5%. The surface humidiflcation of the sheet is accompanied by a reswelling of the surface fibers; and, since redrying is carried on under temperature and time conditions to preserve the surface fibers 20 with residual moisture and short of fully shrunken and densified state, all -ofthe sheet fibers are capable of being acetylated' with that degree of uniformity conducive to practically complete solubility of the cellulose acetate. Or the 26 initial drying of the sheet may be conducted in its last stage in an .atmosphere of regulated humidity and elevated temperature to avoid drying or dehydration of the sheet surfaces or surfacefibers beyond the desired point. Thus, the

30 last stage drying of the sheet may take place as. it is passing continuously in festoon form through 'a chamber at appropriately elevated temperature being supplied with a fog or fine sprays of water, steam, or a mixture of water 35 vapor and air and thereby kept under such humidity conditions as to ensure the desired surface moisture content in the sheet emerging from the chamber.

Another method of restoring'the sheet surfaces 40 or surface fibers m the desired acetylating activity is'to spray them with an aqueous solution of water-miscible organic liquid of high 'boiling point, such as glycerine, ethylene glycol, lactic acid, acetic acid, or the like. Such an aqueous 45 solutions reswells the surface fibers and, by reason ofits higher than 212 F. boiling point, clings' to its last portions 'of water even when the surface-humidified sheet is redried at temperatures higher than 212 1''. but below thatof the organic '50 liquid.

Accordingly, when such aqueous 'solu-' tion is applied to the sheet surfaces, it becomes possible to redry the sheet at temperatures higher than 212' I". while at the same time leaving the surface fibers in the desired hydrated or 55 activated condition. It might beobserved that lactic acid solution is especially effective as an activating agent. Indeed, its ability to activate cellulose fibers for acetylating reaction appears to arise from its chemical action on the fibers in so addition to itsretention in the sheet surface of residual water during redrying of the sheet at temperatures below the boiling point. w Still another way of activating the sheet sin- I faces is to spray thereon reagents whose action as on the fibers is essentially chemical rather than physical. I have found that solutions of hydrogen peroxide, preferably solutions of at least about 0.5% strength, are especially efiective in 1 transforming overdried sheet surfaces to the de- 7 sired acetylating activity and further that a sheet 'wetted on its surfaces with such hydrogen peroxide solution becomes so activated thereat that it may be redried without particular regard to redry'ing conditions or residual moisture content I, in its redried surfaces. In such case, the hydrogen peroxide modifies thc cellulose fiber chemically; and this modification may be attended by such increased acetylating activity of the fiber that the preservation of particular physical quality therein is unnecessary, wherefore, the final or redried sheet may be practically devoid of residual water at its surfaces.

I shall now give specific illustrations of the practice hereof. A comparatively thick woodpulp sheet, forv instance, one of about 0.05 to 10 0.10 inch thickness, whose surface portions have been overdried and thus rendered passive toward acetylation, may be delivered by a pulp drier at a total moisture content of about 2% based on bone-dry pulp. The surface portions of the sheet may be bone-dry so that such moisture is largely localized in the interior or central portion of the sheet. Such non-uniform moisture distribution throughout the sheet may be the result of the previous drying of the=sheet on drier-drum surfaces-heated to temperatures of about 260 to 300 F. Such a dried sheet may be progressively sprayed on both its surfaces, either as delivered from the pulp drier or from a roll accumulation,

with water until it contains about 15% to 20% of moisture, based on the weight of the bone- .drysheet as a whole. The sprayedmoisture, which may be emitted from water-atomizers or steam jets, tends to remain concentrated largely on the surface portions of the sheet, whereit is needed for activating the fibers. in the short period of time elapsing between humidification and exposure of the sheet to drying influence. Thus, the surface-humidified sheet may be brought into contact with hot air or hot drum surfaces only a few seconds after or almost immediately after the surface-humidifying treatment, in which case redrying takes place .before the' surface moisture has had opportunity to diffuse materially into the interior of the sheet 0 and to become equalized throughout the sheet. Redrying may be effected at a temperature sufficiently below 212 E, say, 150 to l'l5..l"., to ensure a redried sheet of the desired acetylating activity throughout, for instance, a sheet con- 5 taining about 2% to 5% moisture substantially uniformly throughout its fibers. In some instances, the humidified sheet may be stored as rolls before it is redried under the appropriate conditions. However, the prompt redrying' of the sheet is generally to be preferred.

In lieu of activating the sheet surfaces with plain water, an aqueous solution containing about 1% to 3% of glycerine, ethylene glycol, lactic acid, or acetic acid may be sprayed onto the sheet 5:;

surfaces so as to infuseinto the surface portions I daily when lactic acid solution is employed, to

produce a final sheet of the' desired acetylating activit'ythroughout. The redried sheet may have a moisture content of about 2% to 5% distributed substantially uniformly therethrough. as

When using an aqueous solution of hydrogen peroxide for 'humidifying the sheet, it is preferable that the solution be of at least about 0.5% strength, for instance, of a strength ranging from about-0.5% to 8%.- The sheet may be sprayed 7 with such solution to acquire about 10% to 30% of its bone-dry wel ht of solution," which is present largely in its ace portions when it is subjected to redrying trea t. The peroxide activates the surface fibers so 2,; 19, 1 1 5 that even when redrying of the sheet is performed at temperatures of 212 F. or higher and for a period of time productive of a redricd sheet containing only about 2% moisture concentrated largely in its interior, such sheet has the desired acetylating reactivity throughout. The hydrogen peroxide additionally improves the color or whiteness of the sheet; and such improvement is reflected in clearer and brighter cellulose acetate solutions prepared froin the cellulose acetate into which the sheet is converted. Indeed, when the wood pulp is of the {appropriate chemical purity, the cellulose acetate solutions prepared from sheeted pulp wetted or humidified with hydrogen peroxide solution are of better color and clarity than cellulose acetate solutions whose cellulose base is one of high grade cotton origin. The hydrogen peroxide leaves no undesirable residues in the sheet. It tends to lower the solution viscosity of the fiber, but, because of the increased acetylating activity ofthe fiber to be realized thereby, it is possible to reduce the amount of sulphuric acid used as a catalyst in the acetylating re- 1 agents and to shorten materially the time and temperature of acetylation and thereby to avoid the formation of cellulose acetate of comparatively low solution viscosity such ,as may be undesirable for some purposes. The use of hydrogen peroxide solution in the treatment of cellulose fiber preparatory to the conversion of the fiber into cellulose derivatives other than cellulose acetate is thesubject of a broader invention disclosed and claimed by me in my application Serial No. 77,611, filed May 2, 1936. Accordingly, I claim herein only those aspects of the use of hydrogen peroxide solution as are pertinent to the invention hereof.

While it is preferable. in usual commercial practice to dry pulp'on a pulp drier as quickly' as possible to the desired residual moisture content by the use of high temperatures, for instance, in contact with drier drum surfaces at a temperature of about 250 to 300 F., so as to realize high output of dried sheeted pulp, 'yet,

as already indicated, such drying results in overdried sheet surfaces requiring re-wetting or humidification and redrying in accordance with the invention 'hereofin order to acquire the desired acetylating activity. In some instances, however, the drying of the sheet as it comes from the wet end of the sheet may be carried out at temperatures of, say, 250 to 300 F. until only about 20% to. 25% moisture, based on the bone-dry weight of fiber, is present in the sheet,whereupon dry- 55 mg of the sheet to a residual moisture content of about 2% to 6%, preferably about 2%, based on the 'bone-dry weight of fiber, may be com pleted at a temperature of about160 to 175 F.

When the final stage of drying is performed, at such'elevated temperature downwards of about .212 FL, the sheet has its residual moisture content more evenly distributed throughout and is hence better adapted'for acetylation. Because even the 20% to 25% moisture content present in the partially'dried sheet, that is, a sheet which has been initially partially dried at temperatures upwards of 212 F., maybe largely concentrated in its interior and have insufiicient opportunity to diffuse into the surface portions during the last or lower-temperature drying stage to restore the surface portions to the desired residual moisture content and attendant acetylating activity,

it may be preferable to spray onto the surfaces of such partially dried sheet containing about 75 20% to 25% moisture additional moisture in the distributed with sufiicient uniformity throughout the sheet so that the desired acetylation results are ensured. The addition of moisture to the partially dried sheet preparatory to last-stage drying may be made in the-form of an aqueous solution of glycerine, ethylene glycol, lactic acid, acetic acid'or hydrogen peroxide, in which case, it is unnecessary to conduct suchlast-stage drying at temperatures below 212 F. Generally speaking, the preferred practice of. the present invention is to dry the sheet completely at temperatures upwards of 212 "F despite the fact that the acetylating activity of the sheet surfaces is impaired, and then to reactivate such surfaces by any one or more ofthe methods hereinbefore described. It will be appreciated, of course, that the sheeted pulp conditioned for acetylation produced by all of these methods may be accumulated as rolls or may be cut into the usual unit sheets of convenient size and the sheets stacked and baled ready for consumption by the acetylation industry. The rolled sheet material or the individual sheets may be cut or reduced to small pieces, chips, or shreds beforebeing subjected to the action of. the acetylating reagents.

The principles of the present invention are applicable to especially good advantage in a woodpulp mill wherein wood pulp is being refined to the degree or purity and whiteness required for the acetylation industry, for instance, an alpha cellulose content of at least about 96%. The preparation of wood pulp of such character generally involves the extraction of non-alpha cellulose impurities from chemically preliberated wood pulp by the action of alkaline liquors. A

.highly satisfactory process for the preparation lulose pulps of other than wood origin to be supplied to the acetylating industry in dried sheet form. It might be remarked that-it is generally desirable to avoid the presence of more than about 2% residual moisture contentin the dried sheets on account of its diluting eifect on the mixed acetylating reagents, whereas, on the other hand, it is desirable that there be some residual moisture throughout the pulp sheets, since, as hereinbefore stressed, the dehydration of the sheets or the surface portions of the sheets to practically bone-dry condition makes for passivity of the dehydrated cellulose toward acetylating reagents. It might be further notedthat through the treatment of the sheet surfaces with aqueous solutions of one or more of such reagents as hydrogen peroxide, lactic acid, acetic acid, etc., it becomes less necessary to control the residual moisture content in the sheet on account of the chemical activation of the surface fibers; and, in such case, it is well to'avoid the retention of more than about 2% moisture in the sheet so as to give the acetylator a form of materlal that requires no dehydration whatever atru his plant preparatory to being brought into contact with the acetylating reagents.

It is possible to performthe humidifying stepof the present invention other than by spraying 5 water or using water vapor or steam, for instance, by the use of so-called kissing rolls serving to hydrate the surface portions of the sheet with a controlled amount of moisture. The moisture supplied to the surface portions of a predried 10 sheet, for instance, a sheet of only about 2% residual moisture\content, whose surface portions are practically bone-dry and have been rendered passive toward acetylation by such overdrying, need not exceed about 30% in order to effect the i5 desired reactivation of such surface portions; and such reactivated state in the surface portions is preserved during the subsequent step of vaporizing the moisture in excess of about the 2% to 6% residuum desired throughout the finished 20 sheet. As already indicated, a total moisture content of about 2% in the sheet is generally preferred; and, while there may be a somewhat greater concentration of moisture in the sheet interior than on its surfaces, nevertheless it is 25 desirable to maintain a residual moisture content of at least 1% in its surface fibers.

I claim:

1. In the production of sheeted cellulose fiber for conversion into cellulose derivatives, that 30 practice which comprises humidifying substantially only the surface of a predried sheet whose surface is practically completely dehydrated; and redrying the sheet under temperature and other conditions controlled to avoid substantially com- I. 40-dehydrated, to an average moisture content of about 10% to 30% and redrying the sheet under temperature and other conditions controlled to avoid residual moisture content of less than 1% or more than about 5% in the surface fibers.

4 3. A process which comprises sheeting cellulose pulp, drying it at a temperature of about 212 to 800 1".. humidifying the dried sheet, and redrying the humidified sheet while retaining at least 1% but not more than about 5% residual moisture 50 in its surface fibers.

4. A process which comprises sheeting cellulose pulp, drying it in contact with surfaces at a temperature of about 212 to 300 F., humidifying the sheet to an average moisture content of 55 about 10% to 30%. redrying the humidified sheet while retaining at least 1% but not more than about 5% residual moisture in its surface fibers, and acetylating the pulp of which the resulting dried sheet is composed. I

5. A process which comprises humidifying a predried sheet of mercerized cellulose pulp and drying the humidified sheet while retaining at least 1% but not more than about 5% residual of which the resulting dried sheet is composed.

7. A process which'comprises humidifying a' predried sheet of wood pulp having an alpha cellulose content of at least about 96% and dry- 1 ing the humidifiedsheet while retaining at least 1% but not more than about residual moisture in its surface fibers.

8. A process. which comprises humidifying a predried sheet. of wood pulp having an alpha cellulose content of at least about 96%, drying 5 the humidified sheet while retaining at least 1% but not more than about 5% residual moisture in its surface fibers, and acetylating the pulp of which the resulting dried sheet is composed.

9. In the production of sheeted cellulose pulp for acetylation involving formation of a sheet from an aqueous pulp suspension and drying of the sheet. that practice which comprises drying the sheet at such elevated temperature as to dehydrate its surface portions practically completely and thus render them passive toward acetylation; and then activating substantially only said surface portions for acetyla'o'ng reaction.

10. In the production of sheeted cellulose pulp for acetylation involving formation of a sheet from an aqueous pulp suspension and drying of the sheet, that practice which comprises drying the sheet 'at such elevated temperature as to dehydrate its surface portions practically completely and thus render them passive toward acetylation; then humidifying substantially only such surface portions to reactivate them for acetylation; and redrying the humidified sheet while retaining sufficient residual moisture in such portions to preserve them in reactivated state.

11. In the production of sheeted cellulose pul involving formation of a sheet from an aqueous pulp suspension and drying of the sheet, that practice which comprises drying the sheet at 3 such elevated temperature as to dehydrate its surface portions practically completely; then humidifying substantially only such surface portions with an aqueous solution of water-miscible-organic liquid having a boiling point higher than 212? F.; and redrying the humidified sheet at a temperature higher than 212 Frbut below the boiling point of said organic liquid.

' 12. In the production of sheeted cellulose pulp involving formation of a sheet from an aqueous pulp suspension and drying of the sheet,.that practice which comprises drying the sheet at such elevated temperature as to dehydrate its surface portions practically. completely; then humidifying substantially only such surface portions with an aqueous solution of lactic acid: and redrying the humidified sheet. v

13. In the production of sheeted cellulose pulp involving formation of a sheet from an aqueous pulp suspension and drying of the sheet, that practice which comprises drying the sheet at such elevated temperature as to dehydrate its surface portions practically completely; then humidifying such surface portions with an aqueous solution of hydrogen peroxide; and redrylng the humidified sheet.

14. In the. production of sheeted cellulose pulp involving formation of a sheet from an aqueous pulp suspension and drying of the sheet, that practice which comprises drying the sheet at such elevated temperature as to dehydrate its surface portions practically completely; then humiditying such surface portions with an aqueous solution of hydrogen peroxide of at least about 0.5% strength; and drying the humidified sheet.

15. In the production of sheeted cellulose pulp for acetylation-invoiving formation of a sheet from aqueous pulp suspension and drying of the sheet, that practice which comprises drying the sheet at a temperature of about 212 to 800' 1''. 7,5

' to based on the dry weight of pulp, interior largely containing and possessing acetylat-ing activity but the sheet so as to dehydrate its surface portions practically completely and thus render them passive toward acetylation; then humidifying substantially only such surface portions to reactivate them for acetylation; and redrying'the humidified sheet at temperatures downwards of 212 F. while retaining sumcient residual moisture in such portions to preserve them in reactivated state.

16. In the production from aqueous pulp suspension of sheeted wood pulp for acetylation involving progressive formation and drying of a wood pulp sheet of a thickness of about 0.03 to 0.10 inch, that practice which comprises progressively drying the sheet at elevated temperature-to a residual moisture content of about 2% the sheet said residual moisture surfaces being so devoid of moisture as to be passive toward acetylation; progressively humidifying the surface portions of the dried sheet to reactivate them for acetylation; and progressively redrying the humidified sheet to a residual moisture content of about 2% to 5%, based on the dry weight of pulp, under conditions to retain such residual moisture with suflicient uniformity pension of sheeted wood pulp surface portions to reactivate them tion;

downward of 212 throughout the sheet to render it acetylatable susbtantially uniformlythroughout.

from aqueous pulp susfor acetylation involving progressive formation and drying of a wood pulp sheet of a thickness of about 0.03 to 0.10 inch, that practice which comprises progressively drying the sheetat a temperature of about 212 to 300 F. to a residual moisture content of about 2% to 5%, based on the dry weight of pulp, the sheet interior largely containing said residual moisture and possessing acetylating activity but the sheet surfaces being so devoid of moisture as to be passive toward acetyiation; progressively spraying about to 30% of moisture, based on the dry weight of pulp, onto said for acetylaand progressively ,redrying the sheet promptly after humidiiication at a temperature F. to a residual moisture content of about 2% to 5%, based on the dry weight of pulp, istributed with sufficient uniformity throughout the sheet to render it acetylatable substantially uniformly throughout.

17. In the production GEORGE A. RICHTER. 

